The cytochrome P-450 dependent monooxygenase system plays a pivotal role in both the detoxification and bioactivation of drugs, environmental contaminants, and other potential chemical toxicants. The balance between detoxification and activation is largely dependent on the relative amounts and activities of different isozymes of cytochrome P-450. The long-term goal of the research proposed in this application is to design irreversible inhibitors of specific isozymes of cytochrome P-450. Such isozyme-specific inhibitors could be used in vivo either 1) diagnostically to assess the tole of the various cytochromes in mediating or protecting against chemical toxicity of 2) therapeutically to redirect the metabolism of xenobiotics from potentially harmful to innocuous pathways. This proposal will focus on determining the mechanism, isozyme selectivity and structural requirements of the suicide inactivation of hepatic cytochromes P-450 by dichloromethyl compounds such as chloramphenicol. Emphasis will first be placed on determining the structural features responsible for the selectivity of chloramphenicol analogs as inactivators of rat liver cytochromes P-450 and on synthesizing specific inhibitors of certain hepatic cytochromes P-450 in the rat and rabbit. Other experiments will be directed towards determining the mechanism by which the heme moiety of the major phenobarbital-inducible isozyme of rat liver cytochrome P-450 is destroyed by certain dichloromethyl compounds. Subsequent studies will focus on assessing the feasibility of using some of the selective inhibitors identified as probes and modulators of the activity of hepatic cytochromes P- 450 in uninduced rats. Finally, the selectivity of chloramphenicol as an inactivator of human liver cytochromes P-450 in vitro will be evaluated. These studies should provide the rational basis for the design of isozyme-specific inhibitors for modulating monooxygenase function in humans.